Distorted depth image - planes slightly concave


I noticed that flat surfaces (IRL) get slightly concave in my depth images. I am using an Astra Pro camera with OpenNI2.

Here are two sample images I took. The first was taken with the OpenNI sample program “SimpleViewer”, having the camera orthogonal to the wall. The center looks slightly concave (or is it some blinding issue ?).

The second one was taken with the camera orthogonal to the ceiling (the black square in the center is a window). I applied some thresholding and we can see that there is a region in the center that is over the threshold value, but my ceiling is actually flat, so the whole surface should disappear at the same time.

(I cannot post it because I am new user, I will try to post it as an answer…)

I also noticed that the corners of the depth image are also distorted a lot (closer to the camera than what they should be), this is not a real problem for me but might be a clue to guess what is the problem.

Is it some kind of lens distortion that I can fix with calibration, or can I correct it with some 3D transformations on the depth data ?

Thanks for the help !

Best regards,

Here is the second picture from the ceiling: